The measurement of physical properties of materials is of great importance in many industries. In the semiconductor industry, for example, it is often necessary to know the dopant level and dopant uniformity in a semiconductor wafer. Similarly, it may be useful to be able to measure the uniformity, quality, or purity of thin films of materials such as insulators, metals, or superconductors. For some applications of superconducting thin films, for example, it would be advantageous to be able to check that the superconducting thin film is of uniform quality and composition. Further, it would be advantageous to monitor the growth of such superconducting thin films. These are just a few of the practical applications of material physical property measurement.
U.S. Pat. No. 5,543,919 to Mumola describes an apparatus and method for measuring with high spatial resolution the thickness of a thin film. The apparatus uses a monochromatic light source to illuminate the thin film and a CCD camera to capture an image of the reflected light. Interference fringe patterns are analyzed to obtain a thickness profile of the entire surface imaged. Multiple exposures at different wavelengths may be used to eliminate thickness ambiguities. Mumolas device is not capable of measuring composition uniformity or many other physical properties of the sample material. The method and apparatus can only be used to measure the thickness of a thin film. A notable feature of this invention is that it does not impose a physical parameter change such as a change in temperature on the sample.
U.S. Pat. No. 5,107,119 to Kimura et al. describes a method and apparatus for measuring physical properties of superconducting thin films. The invention works by passing far-infrared light through the thin film and analyzing the spectrum of the light transmitted. Some notable features of Kimuras apparatus are: 1) it measures light transmittance, not reflectance, 2) it is not an imaging technique, 3) it uses a broadband, not monochromatic, infrared source, and 4) it is limited for use with superconducting materials. Kimura's apparatus is not capable of measuring the composition and composition uniformity of a variety of materials.
U.S. Pat. No. 5,490,728 to Scheitinger et al. describes methods for measuring physical properties of a surface with noncontact optical techniques. The surface is illuminated with white light having a temporal intensity ripple. The spectrum and intensity of the light reflected by the surface and the thermal radiation emitted by the surface are measured. Precise determinations of emissivity, reflectivity, temperature, changing surface composition, the existence of any layer formed on the surface and its thickness are all possible from this measurement. Scheitingers invention is particularly applicable to semiconductor wafer processing and metal processing. Scheitinger does not image the surface to provide a spatial map of the characteristics measured. Therefore, the method can be of limited use for some applications. Three important notable features of Scheitingers invention are: 1) it does not compare multiple images of the surface, 2) the incident light is time-varying, and 3) the incident light is broadband, not monochromatic.
U.S. Pat. No. 5,439,291 to Reading describes a technique for determining physical properties of a sample using thermal modulation techniques. Two identical samples are used, with one experiencing a linear temperature ramp and the other experiencing the same ramp with a temperature oscillation imposed. A chopped light source can be used to provide the energy necessary for the temperature oscillation. Thermocouples attached to each sample measure the temperature of each sample, which is the diagnostic means. This invention does not image the samples. Light is only used as a radiation source to heat the temperature-modulated sample, and is not used as a diagnostic means.
The prior art devices do not provide a means for imaging the composition or other physical properties of a sample surface. The dopant level of semiconductor wafers, for example, cannot be imaged using the prior art techniques. Also, the composition and composition uniformity of superconducting thin films cannot be imaged.
Therefore, there exists a need for a technique for imaging physical properties such as composition of a material sample using noncontact techniques. Further, it would be advantageous for this technique to be applicable to many different types of materials.